The present invention relates to a filter device for microfiltration of oil, in particular for use in engines, automatic transmissions and hydraulic systems, the filter device comprising a tubular filter cartridge, through which medium can flow in the transverse direction and which is microporous, with open ends; a filter housing which encloses a filter chamber in which the filter cartridge is accommodated or can be accommodated in such a manner that the filter chamber is divided into a first space which lies outside the filter cartridge and a second space which lies inside the filter cartridge, which spaces are in communication with one another via the tubular wall, which acts as a filter, of the filter cartridge; the filter housing being provided with a first end closure element and a second end closure element for closing off the open ends of the filter cartridge and at least partially or completely overlapping the end walls of the filter cartridge; and the filter housing being provided with holding means for clamping the filter cartridge between the end closure elements under prestress.
A filter device of this nature is known to German Patent DE-3,421,399 C2. The structure which is known from this patent uses a setscrew which prestresses the two end closure elements towards one another and thus clamps the tubular filter cartridge between the end closure elements.
Other designs are also known, for example designs in which the end walls of the filter housing are also the end closure elements for closing off the end walls of the filter cartridge, in which case one end wall is a fixed end wall and the other end wall is a screw cap which, in order for the filter cartridge to be replaced, can be unscrewed from the housing and then screwed back onto the housing. As a result of this screw cap being tightened to a sufficient extent, the filter cartridge is then clamped between the filter-housing end walls, which act as end closure elements.
In practice, however, it has been found that filter devices of this nature still allow some dirt to pass through. The applicant has ascertained that this xe2x80x9cleakagexe2x80x9d is attributable to slight changes in length which the filter cartridge experiences when the installation in which the filter device is fitted is started up and stopped. Specifically, the applicant has discovered the following:
During operation, the oil which is to be cleaned in the filter is fed to the outside of the filter cartridge at high pressure. Under the influence of the high pressure, the oil will then pass through the filter cartridge and, in the process, contamination will be removed. In filter cartridges of his nature, paper-like materials, felt-like materials and also glass-fibre materials are generally used as the filter material, in particular for microfilters. In view of the high pressure, these filter cartridges are often supported on the inside of the tube if the external pressure is higher than the internal pressure or on the outside of the tube if the internal pressure is higher than the external pressure. The relatively deformable filter material will then be compressed under the influence of the high pressure and will cause the length of the filter cartridge to increase slightly. When the installation in which the filter device is fitted is then stopped, and the oil pressure is therefore reduced again to a low pressure or to zero, this filter material will expand again in the transverse direction, and the length of the filter cartridge will decrease slightly. As a result, in this state, a small gap is formed between the end wall of the filter cartridge and the respective end closure element. When the installation in which the filter device is fitted is then started up again, and the oil is set in motion once again, with the result that the pressure rises again, there will initially be a moment at which contamination is forced through this gap from one side of the filter to the other side of the filter. Although this phenomenon is only transient, it nevertheless represents a not insignificant shortcoming of the filter device which, in the medium to long term, leads to damage to the installation in which the filter device is fitted.
Now that the cause of this long-standing problem has been detected, the object of the present invention is to provide a filter device of the type mentioned in the introduction in which xe2x80x9cleakagexe2x80x9d through the gap between the end wall of the filter cartridge and an associated end closure element is prevented or at least reduced. According to the present invention, the problem which has been recognized by the applicant can be solved by two different routes, which two different routes can also be followed in combination with one another, in which case they lead to an even better result than if these two routes are followed separately.
According to the first route, the problem which has been recognized by the applicant is overcome by the fact that at least one, preferably both, of the end closure elements, on its/their surface which faces towards the respective end wall of the filter cartridge, is/are provided with at least one rib which extends in the transverse circumferential direction of the filter cartridge. Under the influence of the holding means, ribs of this nature are pressed into the end wall of the filter cartridge and interrupt the gap which may be formed when the filter device ceases to operate, or at least when the oil pressure is eliminated. It will be clear that this measure stops fine contamination from being forced through the gap when the installation is started up. Since the pressure on the filter cartridge fills when the installation is stopped, the filter cartridge will moreover also tend to expand slightly in the transverse direction, in which case, if the ribs have already been retracted slightly from the end walls of the filter cartridge, it will still be able to adjoin these ribs under the influence of the reduction in length of the filter cartridge. However, it should be noted that even if, in the event of the rib which has been pressed into the end wall of the filter cartridge coming slightly free from the end wall and of the filter material no longer bearing fully against the rib, the gap which is formed when the oil pressure is eliminated will at any rate wind around the rib at the location of the rib, and this fact alone makes it difficult for dirt to be pressed through the gap.
In order to interrupt the gap along the entire end wall, it is advantageous according to the invention if the at least one rib extends without interruption and in the form of a continuous ring over the said end closure element.
To allow the rib to be pressed into the end wall of the filter cartridge with relative ease, it is advantageous according to the invention if the said at least one rib ends in a pointed edge which is directed towards the respective end wall of the filter cartridge. In this case, it is particularly advantageous if this pointed edge is designed as a sharp edge, as a cuffing edge, as it were.
To ensure that the at least one rib continues to extend all the way to the end wall of the filter cartridge at all times, it is advantageous, according to the invention, if the height of the at least one rib, in the longitudinal direction of the filter cartridge, is greater than a predetermined change in length which the filter cartridge will undergo at a predetermined maximum operating pressure of the oil.
According to a second route, which can therefore be regarded as being entirely separate from hte first route, but is preferably to be regarded in combination with the first route, the problem which the applicant has recognized is solved by the fact that the holding means comprise a spring which acts with prestress on one of the end closure elements, in such a manner that this closure element follows the end wall which it adjoins in the event of longitudinal expansion and/or longitudinal contraction of the filter cartridge. It is thus possible to ensure that the two closure elements continue to bear in a sealed manner against the end walls at all times, under the action of the prestressed spring.
According to an advantageous embodiment of the invention, the spring is a compression spring which is accommodated, with prestress, between, on the one hand, that side of the said one end closure element which faces away from the filter cartridge and, on the other hand, the wall of the filter housing.
According to an advantageous embodiment of the invention, in this case, the prestress of the spring is at least 3 kg/cm, preferably at least 5 kg/cm, and at most 9 kg/cm, preferably at most 7 kg/cm.
In both solutions (routes) according to the invention, it is advantageous, according to the invention, if the end closure elements are provided, on their side which faces towards the filter cartridge, with a sealing plug which can be fitted tightly into the interior of the filter cartridge. This, on the one hand, ensures that the filter cartridge is securely fixed in the chamber of the filter housing and, on the other hand, provides yet a further seal between the inner wall of the filter cartridge and the sealing plug at the location of the open ends.
According to a further advantageous embodiment of the invention, the filter cartridge comprises a perforated winding core with paper, in particular filter paper, wound onto it.